Use boolean selector syntax for tags.

doc/package_config.md

 Each package may include a configuration file that applies to the package as a
 whole. This file can be used to provide custom defaults for various options, to
 define configuration for multiple files, and more.
 
 The file is named `dart_test.yaml` and lives at the root of the package, next to
 the package's pubspec. Like the pubspec, it's a [YAML][] file. Here's an
 example:
 
 [YAML]: http://yaml.org/
 
@@ skipping 164 matching lines @@
 "expanded", or "json" (although why anyone would want to default to JSON is
 beyond me). It defaults to "expanded" on Windows and "compact" everywhere else.
 
 ```yaml
 reporter: expanded
 ```
 
 ## Configuring Tags
 
 The `tag` field can be used to apply [test configuration](#test-configuration)
-to all tests [with a given tag][tagging tests]. It takes a map from tag names to
-configuration maps. These configuration maps are just like the top level of the
-configuration file, except that they may not contain
+to all tests [with a given tag][tagging tests] or set of tags. It takes a map
+from tag selectors to configuration maps. These configuration maps are just like
+the top level of the configuration file, except that they may not contain
 [runner configuration](#runner-configuration).
 
 [tagging tests]: https://github.com/dart-lang/test/blob/master/README.md#tagging-tests
 
 ```yaml
 tags:
   # Integration tests need more time to run.
   integration:
     timeout: 1m
 ```
 
 Tags may also have no configuration associated with them. The test runner prints
 a warning whenever it encounters a tag it doesn't recognize, and this the best
 way to tell it that a tag exists.
 
 ```yaml
 # We occasionally want to use --tags or --exclude-tags on these tags.
 tags:
   # A test that spawns a browser.
   browser:
 
   # A test that needs Ruby installed.
   ruby:
 ```
 
+You can also use [boolean selector syntax][] to define configuration that
+involves multiple tags. For example:
+
+[boolean selector syntax]: https://github.com/dart-lang/boolean_selector/blob/master/README.md
+
+```yaml
+tags:
+  # Tests that invoke sub-processes tend to be a little slower.
+  ruby || python:
+    timeout: 1.5x
+```
+
 Tag configuration is applied at whatever level the tag appears—so if a group is
 tagged as `integration`, its timeout will take precedence over the suite's
 timeout but not any tests'. If the group itself had a timeout declared, the
 group's explicit timeout would take precedence over the tag.
 
 If multiple tags appear at the same level, and they have conflicting
 configurations, the test runner *does not guarantee* what order they'll be
 resolved in. In practice, conflicting configuration is pretty unlikely and it's
 easy to just explicitly specify what you want on the test itself.
 
@@ skipping 11 matching lines @@
   browser:
     timeout: 2x
 
   # Tests that spawn specific browsers. These automatically get the browser tag
   # as well.
   chrome: {add_tags: [browser]}
   firefox: {add_tags: [browser]}
   safari: {add_tags: [browser]}
   ie: {add_tags: [browser]}
 ```